Radar system



M. A. MEYER 2,982,956

RADAR SYSTEM 5 Sheets-Sheet 1 /N VENTOR MAURICE A. MEYER ATTnP/IPY May 2, 1961 original Filed sept. 10, 1956 May 2, 1961 M. A. MEYER 2,982,956

RADAR SYSTEM Original Filed Sept. l0, 1956 5 Sheets-Sheet 2 MAGIC 985| MC 51 MC 5IMC Y TEE F|| TER p LOCAL SOURCE MlXER 9749MC oSG. SIGNAL GATE FREQ. SBOOMG w R t SE'ECTOR STALO DPR/IDR GENERRTOR i9,

951 9 Mc MAGIC SBOSMG 20S Mo SOURCE TEE FII-TER l* TRANSMTTTED 941 g'fslMC x SIGNAL COUNTER CHAIN lol |0210 STAGE STAGE STAGE STAGE STAGE Y Y V GATE GATE GATE GATE GATE I I 2 3 T 4 5 f-|oe 3 L BUFFER 2 4 GATES SMC lo 5 |O8\ SOURCE ALTIMETER V2 FLIP IFLOP o GASURIEMG |05j |03 S R |07/ los /NVE/vron MAUR ICE A. MEYER FIG. 3

By /wzg v A TTORNEY May 2, l961 M. A. MEYER 2,982,956

RADAR SYSTEM Original Filed Sept. 10, 1956 5 She ts Sh t 3 e ee mI-II-ImITImI-ITTI-ITTI-II-IVTI-II-TI-I AII II II II II II II ILJI ILII II II II II IESTAGE' B STAGE 2 C STAGE 5 D STAGE 4 E L STAGE 5 GATE 3 F L M M L OUTPUT G V* FLIP FI OP I S OUTPUT L I FLIP FLOP H l R OUTPUT J .IIIIIIIIIIIIIIIIIIIIIIIIII l III.IIIIIIIIIIIIIIIIIIIIIII RADIATED E SIGNAI I A A I I OOAI. Osc. K IIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIUIIHIHI SIGNAL TIME- 'INVENTOR MAURICE A. MEYER States Patented May 2, 1961 2,982,956 RADAR SYSTEM Maurice A. Meyer, Natick, Mass., assigner -to Laboratory for Electronics, Inc., Boston, Mass., a corporation of Delaware l Original application Sept. 10, 1956, Ser. No. 610,444.

Divided and this application Sept. 1l), 1956, Ser. No.

17 Claims. (Cl. 343-8) The present invention relates in general to pulsed radar systems and in particular to a pulsed Doppler radar system having a pulse repetition frequency which is controlled by an altimeter. Utilization of the inventive concepts in an airborne Doppler radar system results in attaining an exceptionally high degree of system sensitivity at all altitudes. The present application is a division `of the co-pending application of Maurice A. Meyer, entitled Doppler Radar System, Serial No. 610,444, tiled September l0, 1956.

In prior art pulsed radar systems of the Doppler and conventional type, it has been the practice to employ a relatively short duty cycle for the transmitter; that is, the transmitter is on for a much shorter time than it is olf. In such systems, when the transmitter is oif, the receiver is on and can respond to that portion of the radiated energy which is returned. However, since the time interval during which reected energy is being received is limited to the duration in which energy is being returned from scatterers of the transmitted pulse, there remains a relatively large period during which the receiver is operative but no energy is :applied thereto. Accordingly, the receiver may respond to noise, thereby reducing overall system sensitivity.

The low duty cycle of the transmitter is especially disadvantageous in connection with Doppler radar systems, for there may be a large time interval during which the receiver is open lbut no useful signal is received. Moreover, if a selected system sensitivity is required, then a pulse of high frequency energy radiated by a low duty cycle system must have a relatively high peak power. The generation of pulses of high peak power requires bulky components, such as a magnetron and dissipation of relatively large amounts of power. The larger power supply required adds to the bulk and weight of the overall system.

,As indicated in the aforesaid co-pending application, the present system utilizes coherent lixed frequency signals whereas the prior art systems were of the noncoherent type wherein depressed beams are oriented forward and rearward, with a pulsed magnetron generating the microwave energy for radiation. Because no coherent local oscillator signal was there available, it was necessary to determine the Doppler frequency shift by mixing the signal returns from the forward and rearward beams to derive a signal with audio frequency components. If the forward beam were radiating upon a hill while the rearward beam were radiating into a valley, the signal return from the for-mer would arrive before that from the latter. If the beams were pencil beams, then there would not be an interval in which simultaneous returns from Iboth beams Ywere available and no Doppler frequency shift would be detected lfor the previously radiated pulse. To avoid this vditiiculty, the prior art systems radiated beams having a wide dimension substanmially along a hyperbola of constant Doppler 'frequency shift. Thus, energy returned from the earth during a time interval much greater than the duration of the transmittedV pulse. This arrangement dictates a requirement for lan increase in the radiated power to atttain a given system sensitivity.

The` coherent arrangement of the present system enables frequency shifts present in the signal return from each beam to be independently detected. Furthermore, pencil beams are radiated to effect :an increase in system sensitivity for a given radiated power, since the use of pencil beamsenables the receiver to be open during time intervals substantially equal to the duration of the transmitted pulse.

There is another disadvantage inherent in prior Iart airborne Doppler radar systems with respect to the pulse repetition frequency. Heretofore, it has been the practice to select a pulse repetition frequency which does not eX- ceed twice the Doppler frequency shift corresponding to the highest expected velocity in order to avoid the undesirable effects of spectrum foldover described in the above-mentioned co-pending application.

Another undesirable effect results from the finite time required for the energy to travel from the aircraft to the earth yand be reflected back again. At higher altitudes, a longer. time is consumed. I-fvtoo high a pulse repetition frequency is chosen, then at some altitudes a reected pulse will be received which overlaps the next transmitted pulse, and the receiver will not be able to respond to all the reilected energy. In fact, at an altitude where the rellected and next radiated pulse completely overlap,

utilization of the reected pulse would be seriously hampered.

This diiliculty is alleviated somewhat in prior art systems by frequency modulating the pulse repetition frequency. However, at high altitudes the dimension of the cross-section of prior art radiated beams, -along a hyperbola of constant Doppler -frequency shift at the surface of the earth, is so great that energy is returned from each transmitted pulse for an interval which exceeds the period between radiated pulses. Consequently, the return signals from consecutively transmitted'pulses overlap. Since they are non-coherent, the overlapping signals combine randomly and appear as noise to the receiver, thereby reducing system sensitivity. Y Y

Accordingly, it is a primary object of the present invention to provide a pulsed radar system which transmits a pulse of the optimum duration for achieving maximum system sensitivity with a given radiated power.

Another object of the invention is the provision of a pulsed radar system having a high degree of sensitivity, and yet utilizing relatively lightweight transmitter components which consume a relatively small amount of power.

A further object of the invention is the provision of a pulsed airborne Doppler radar system of high sensitivity, yet compact and lightweight and arranged to operate at maximum sensitivity within selected altitude ranges.

Still another object of the invention is the provision of a pulsed Doppler radar system in accordance with the above objects which is capable of responding and utilizing returned Doppler frequency-shifted signals even when substantially at ground level. l

In a broad form, there is provided according to the invention la radar system having meansY for radiating bursts of microwave energy during lirst time intervals and receiving means capable of responding to that portion of the microwave energy which is reflected. The receiver is rendered sensitive to the rellected energy only when microwave energy is not being radiated during second time intervals. The rst and secondtime intervals alternate and are of substantially equal duration. In another aspect of the invention, means sensitive to the Vdistance between the radiating means and the reilecting surface are provided for controlling the repetition frequency of the radiated high frequency bursts or pulses in ac cordance with the distance sensed, the closer the reflecting surface, thehigher the pulse repetition frequency.

' Ina particular form which the invention takes in a spaanse 1 f pulsed airborne Doppler radar system wherein the retlected energy returns from the surface of the earth, the distance sensitive means is an altimeter which effects stepwise changes, in ,the` pulse repetition frequency as altitude ,changes at selected levels are sensed. Preferably, this pulse repetition frequencyis selected to be a sub-harmonic of a fixed frequency signal utilizedv elsewhere in the receiving system, thereby preventing 'unwanted low frequency signals, which might interfere with the received Doppler frequency shifts, from being introduced into thereceiving system.

Other features, objects and advantages of the invention will become apparent `from the following specification when read in connection with the accompanying drawing in which:

Fig. 1 is a block `diagram of a representative embodiment of an airborne Doppler radar navigational system;

Fig. 2 is a block diagram of a preferred embodiment of the microwave transmitter of Fig. l;

Fig. 3 is a block diagram of the novel altimeter controlled gate frequency selector and generator; and

Fig. 4 is a graphical representation of signal waveforms plotted as a function of time and pertinent to the understanding of the apparatus illustrated in Figs. 2 and 3.

With reference now to the drawing and more particularly Fig. 1 thereof, a microwave lens 13 is energized by radiated energy from conical horns 14, 15 and 16, the latter horns` being coupled to microwave converter 17 and microwave transmitter 18 by the directional couplers and power dividers 21. Transmitter 1S generates a trans- Initted signal of frequency fo and local oscillator signal of frequency fm during alternate mutually exclusive intervals in response to gating pulses from gate generator 22, the frequency of this signal being controlled by altimeter 23. Converters 17, energized by the local oscillator signal, provides output signals for application to `I.-F. strips 24 which are also energized by a .pair of fixed frequency signals to provide an output signal to carrier elimination filters 25, displaced in the frequency spectrum from the input signal, but retaining the Doppler frequency shifts. gized by fixed frequency signals that are utilized as carrier signals upon which the Doppler frequency shifted signals are modulated. One output from filters 25 is applied to a frequency doubler 26 whose output is `applied to a mixer 27.` The other two output signals from filters 2S are applied to mixer 28 to provide sum and difference frequency signals,.the sum` signal being applied to mixers 27 and 31 while the difference frequency signal is directly applied to one channel of the trackers 32. The other input to .mixer 31 is a fixed frequency signal to effect anoutput from the latter mixer which includes the desired Doppler frequency shifted signals disposed about a carrier signal, enabling the trackers to respond to the Doppler frequency shifts. The mixer 31 output signal is applied as a second signal input to trackers 32. The third signal for application to the-tracker is derived 'from the output of mixer 27.

The output of the trackers includes signalstwhose frequency shifts are proportional to the three generalized Doppler variables Dx, Dy and DZ discussed in detail in the aforesaid parent application, together with a polarity indication for each variable to indicate the sense of the associated `Doppler shift. The signals are applied to the base line computer 33 which also receives signals from shaft-to-digital converters 34, indicative of pitch angle, roll angle, and the sine and cosine of the aircraftazimuthal heading angle. 'I'he shaft-to-digital converters 34 couple to the computer in digital form, the analog information derived from roll and pitch reference 3S and heading reference 36.A The output of base line computer 33 energizes an alongcourse counter` 37, which indicates the` distance .traveled along the course from the starting `point or other reference point, a cross course counter `tlwhich indicatesthe magnitude and direction ofdeviation across The latter filters are also ener-y operation will be described.' When gate generator 22 renders microwave transmitter 18 operative for the generation of amicrowave signal of frequency fo, the latter signal is coupled through power dividers and directional couplers 21 to` each conical horn 14, 15 and 16 which respectively radiate beams through lens 13 which are focused into pencil beams by the lens.

Energy returned from the three beams is focused by the lens upon the respective horns `from which the energy emanated. The directional couplers 21 direct the returned energy, which includesthe transmitted `frequency fo plus the Doppler frequency shifts fdl, fdz, fda from the beams respectivelyassociated withhorns 14, 15 and 16 to microwave converter 17. In microwave converter 17, the three received signals are mixed with a local oscillator signal to provide the signals with the Doppler frequency shifts, transposed in frequency` about a 42 mc. I.-F. frequency as indicated, for amplification by respective channels in I.-F. strips 24. Fixed frequency signals of 51 mc. and 9.5 me. are also applied to the latter strips and the difference frequency signal is mixed with the 42 mc. signalto provide outputs which include the Doppler frequency shifts about 500 kc.

As indicated above, a pulsed system. is normally arranged so that `the receiver is olf when the transmitter is on. Thus, the 50() kc. carrier signal is usually not present. The exception occurs at very low altitudes when the pulse repetition frequency is at its highest value. Since energy from transmitted pulses returns almost instantaneously, the receiver is deliberately rendered operational during a portion of the interval in which a pulse is transmitted. During this interval, 50() kc. carrier signal is present in the I.-F. strips output signal. However, the proximity of the aircraft to the ground results in a signal return of sufiicient strength to overcome the effects of carrier leakage after selective filtering by carrier elimination filters 42.

The signals from I.-F. strips 24 are applied to the carrier elimination filters 25. Each filter is preferably of the type described in the `co-pending application of M. A. Meyer, entitled Selective Circuit, Serial No. 329,803, filed January 6, 1953, and are as illustrated in Fig. 1 thereof withrespect to filters having fdl and fdz `in the outputs. However, since it is desired that the signal output having the -fds Doppler component be `relatively close to 200 kc., the single side band modulator 25 in Fig. l of the aforesaid application is energized by quadrature components of a 200 kc. fixed frequency signal instead of the reference signal quadrature components as indicated therein.

The signal component containing fdl is applied to doubler 26 to provide an output signal having a frequency component of lOOOfkc. -l-Zfdl. Theother two output signals from the carrier elimination filters, having components including fdz and fds about 500 and 200102. respective1y,vare applied to mixer 28 to provide a difference frequency signal of 300 kc. -l-fdg--fd3 which is applied to one input of the trackers 32. The sum signal ,from mixer 28, having 700 kc. -f-fdg-l-fda is applied to mixer 27, and the difference frequency output therefrom, 300 kc. --.Zfdl-fdZ-fdm applied to another input of the trackers 32. The sum signal from mixer 2S is also applied to mixer 31, which has a second input energized by a 400 kc. fixed frequency signal. The difference frequency signal therefrom is applied as the remaining input signal to the trackers `32.

With reference to Fig. 2, a preferred form of microwave transmitter `18` of Fig. l is depicted in block diagram form. While conventional microwave signal `sources `may be used ito generate the radiated and local Y quency source.

- oscillator signals, the preferred embodiment ofvmicrowave transmitter 18 has features which are especially advantageous in connection with Doppler navigational systems. 'I'hese advantages will be better understood after the discussion of the arrangement of the transmitter and its mode of operation. Microwave transmitter 18 is seen to comprise a stable local oscillator 91 which energizes magic tee mixers 93 and 94 through a power divider 92. Mixers 93 and 94 are also energized by 5l mc. and 9 mc. sources 95 and 96 respectively. The latter signal sources emit signals during alternating mutually exclusive time intervals in accordance with a gating signal from gate frequency selector and generator 22.` The outputs of mixers 93 and 94 are applied to filters 97 and 98 respectively, the output signals from the latter filters being applied to microwave converter 17 and directional couplers and power dividers 21 respectively of Fig. '1. Stalo 91 is a stable microwave oscillator preferably of the type wherein a servo control system, which includes a discriminator cavity, maintains the oscillator frequency at substantially the center frequency of the cavity. Other stable oscillators, such as the type employing a relatively ylow frequency crystal oscillator energizing a chain of frequency multipliers may also serve as the stable local oscillator.

In this example, the output signal from the stable local oscillator is a 9800 mc. microwave signal and is applied through microwave coupling means to power divider 92 which channels portions of the input power through microwave coupling means to magic tee mixers 93 and 94. Although other mixing means may be employed, each mixer is preferably of the type employing semiconductor diodes in a magic tee arrangement which precludes energy from being coupled back to power divider 92. When source 95 responds to the gating signal from gate generator 22 with a 51 mc. output signal, mixer 93 is also energized by the latter to provide an output signal which includes sum and diiference frequency signals of 9851 mc. and 9749 mc. respectively. A filter 97 rejects all but the 9851 mc. signal and the latter serves as the local oscillator signal for application toy a power divider 21 in Fig. 1. When the signal from gate generator Z2 maintains source 95 in the inactive state, the only output from mixer 93 is a 9800 mc. signal which -is rejected by lter 97; hence, there is no local oscillator signal and microwave converter 17 (Fig. 1) is effectively inoperative. Accordingly, receiving apparatus, which includes converters 17 and I.F. strips 24, is then insensitive to received signals. To more completely desensitize the receiving apparatus during the transmitting interval, the 51 mc. source 95 is coupled to terminal 19 of the L-F. strips 24 in Fig. 1, there normally being no 51 mc. signal then applied to terminal 19 during the interval a pulse is transmitted. A further result is a reduction of noise to signal vration of substantially 3 db because thermal noise at the inputfcircuits of the I.F. strip is eliminated during these intervals.

When the signal from gate generator 22 activates source 96, mixer 94 is also energized by a 9 mc. signal to provide a signal output which includes sum and difference frequency signals of 9809 mc. and 9791 mc. respectively. Filter v98 rejects substantially all but the 9809 mc. signal to provide a transmitted signal of 9809 mc. at the output which is applied to a power divider 21 in Fig. l. When the gating signal from generator 22, disables signal source 96, the only output signal from mixer 94 is a 9800 signal which is rejected by filter 98. No signal is transmitted during this interval.

It is seen that this novel arrangement provides the de sired alternate operation of transmitter and receiver at microwave frequencies by controlling the emission of relatively low frequency signals. Stalo 91 continues to emit at all times; hence, no stability problems are presented with respect to the primary microwave signal fre- It is relatively easy to gate the 5l mc.

and 9 mc. sources without affecting the frequency stability f their output signal. Thus, two stable microwave signals are suppliedwhose frequency dierence is the desired high frequency of the receiver I.-F. strips. Since both signals are derived from Stalo 9'1 any drift in the output frequency of the latter causes no change in the diEerence frequency signal. The stability of the latter is dependent only on the Stability of the 9 and 51 mc. signal sources, which frequencies may be controlled within tight tolerances by utilizing well-known crystal oscillator techniques.

As indicated above, the preferred system includes a time-shared transmitter-receiver; that is, when the transmitter is on the receiver is off and vice Iversa. Iltis type of operation effects increased system sensitivity. With C.W. Doppler systems the return signal must be high enough to override carrier leakage signals from the transmitter, but with the system described herein the receiver is operative during intervals when no carrier signal is radiated; hence, substantially all the gain of the receiver .may be utilized for responding to the reected signal.

The particular embodiment preferred for effecting this duplexer type of arrangement utilizes a stable microwave signal source which continually generates a primary microwave signal whose frequency is dierent from that of the transmitted signal, thus enabling the latter source to remain on at all times, the receiver being insensitive to its output frequency, even though portions might leak to the receiver.v

As indicated above, another feature of the present system is the utilization of coherent fixed frequency signals. The signals of Vfrequency 200 kc., 500 kc., 700 kc., 9 mc., 9.5 mc. and 51 mc. are all generated from the same basic timing oscillator source by utilizing a combination of harmonic generators and mixers of the type well known in the art. VSince both the local oscillator and transmitted signals are derivved by combining the same stable microwave signal with one of the coherently generated signals, the transmitted signal and all signals in the receiving system are coherent; hence, despite the frequency translation of the returned Doppler frequencyshifted spectra within the receiving system, precise retention of the Doppler frequency shifts relative to an appropriate reference frequency is readily obtained.

` With 'the coherent arrangement of the present system,

frequency shifts present in the signal return from each beam can be indpendently detected. As a result, pencil beams may be radiated to effect an increase in system sensitivity for a given radiated power. A further advantage is that the Doppler frequency-shifted spectra may be tracked at relatively high frequencies, eliminating the problems encountered in connection with spectrum foldover described in the parent application.

A feature of the present system is the utilization of a 50% duty cycle; that is, the duration of each radiated pulse is substantially equal to the Vtime interval between pulses. Accordingly, reected energy is returned to the receiver for a longer period of time as compared with prior art low duty cycle pulsed radar systems where the interval between'radiated pulses greatly exceeds the pulse duration. Furthermore, by controlling the pulse repetition rate in accordance with the aircraft altitude so that as the transmitted pulse ends, the reflected energy from the leading edge thereof returns to the aircraft, the receiver may operate at maximum sensitivity while responding to substantially the entire retiected pulse.

Another advantage of the 50% duty cycle of the present invention is the nature of the frequency spectrum thereby radiated. Most of the energy is in sidebands relatively close to the carrier frequency. Accordingly, even the prior art systems, which track at relatively low frequencies, would be supplied with more lowV frequency en- Yergy in the signal return if a high duty Acycle were employed, therebyincreasing'system sensitivity. The relatively narrow radiated pulses of the prior art systems have aspectral distribution wherein a substantial portion of the radiated energy is in the higher order sidebands, which is all discarded by the low passfilter arrangement used therein to alleviate ther ambiguity `problem.discussed `inathe aforesaid-copending application in` connection with` spectrum foldover.

Operation fof the aforesaid system will be better understood from the following discussion of the system block diagram in Fig. AEl, and the signal waveformsgraphically represented as functions of time inFig. 4. With reference to Fig. 3, there is illustrated in block diagram form, gate frequency selector and generator 22 of Figs. 1 and 2 arranged-to cooperate with altimeter 23 of Fig. 1. A counter chain 101 is energized on terminal 102by the 500 kc. signalv utilized elsewhere in the system illustrated in Fig. l. Thefoutput -of each counter stage is coupled to a gate whose other input is connected to a terminaly on switch 103, each terminal and gate bearing a number which corresponds tothe associated counter stage. The arm ofswitch 104 is coupled to a source of positive potential at terminal 105 and actuated by the shaft yof altimeter 23. The outputs of the gates are coupled to buffer 106 which in turn energizes flip-op 107. The' S output of ilip-iop 107 at terminal 108 is coupled to the 51 megacycle source 95 in Fig. 11, while the R output thereof on terminal 109 is coupled tothe 9 megacycle source 96. y

Referring to the signal waveforms of Fig. 4, the mod of operationof the system offFig. 3 will be described. Counter stage 101 is energized at stage 1 by the SOOkc. signal `utilized elsewhere in the system. Stage 1 responds to this input signal with a plate signal waveform illustrated `in Fig. 4A. The remaining stages respond to the signals from the preceding stage to vprovide plate signal waveforms illustrated in Figs. 4B, 4C, 4D and 4E. Each of these plate waveforms is differentiated and applied to an associatedtgate. The arrn of switch 104 is actuated by movement of the shaft of altimeter 23, the system being arranged so that arm 104 connects terminal 105 to switch position 1 when the altimeter indicates 0-2000 feet, to switch terminal 2 when 200G-4000 feet is indicated, to switch terminal 3 when 400G-8000 feet is indicated, to switch terminal 4 when 8000-16,000 feet is indicated, and to switch terminal 5 when readings above 16,000 feet are indicated. When a switch terminal is connected to terminal 105, a corresponding gate is activated and output pulses therefrom are coupled to buffer 106 which in turn couples `pulses to flip flop 107.V For example, with arm `104 connected to switch terminal 3 as illustrated, the gate 3 output pulses illustrated in Fig. 4F are coupled `through buffer 106 to flip flop 107, the latter responding by providing as an outputsignalon` terminals 108 and 109 the plate waveforms from the S and R sections respectively illustrated in Figs. 4G and 4H. The latter two waveforms are of opposite phase and are respectively applied to the 9 mc. source 96 and the 51 mc. source 95 to control their respective outputs. In response to the two gating signals from flip op 107, the output signals from lters 98 and 97 of Fig. 2V are as illustrated in Figs. 4I and 4K respectively. Thus, bursts of a signal for radiation and local oscillator signal are generated for equal durations, but during mutually exclusive alternating time intervals.

The reason for varying the pulse repetition rate in steps is to avoid unwanted modulation products in the received signal. As indicated in Fig. 1, the input signals to the carrier elimination filters includes a 500 `kc. component. Since the generated microwave signals are pulsed at a sub-harmonic of 500 kc., the received signal also contains a 500 kc. harmonic of the pulse repetition frequency. However, since the gating signal is derived from the same 500 kc. source which energizes the rest of the system, the harmonics at -500 kc. are in phase with other 500 kc. signals present and introduce `no `additional frequency-shifted components which might erroneously be detected as Doppler shifts. It has been discovered .8 that utilization ,of the indicated technique of halving the pulse repetition `rate when the indicated altitude is doubled results i in adequate` system sensitivity.`

` Although thisgating system has been described in connectionwith `:a conventional electron tube ip op counter chain-anday vacuum tubeip flop 107, the novel concepts 'may be embodied, utilizing other bistable circuits such as transistorand/or magnetic core circuits which perform similar functions and ,altimeter information may be supplied from any suitable altitude indicating device.

At low altitudes where the transmitted and returned signals overlap and it is desired to render the receiver operative during the intervals in which a pulse is transmitted, means maybe provided for coupling a source of positive gating potential to the '51. mc. source 95 continuously instead of the signal waveform on output `terminal 109. The latter'means `may be manually operated or responsive toan altimeter indication less than a predetermined value.

In connection with `the foregoing description of a preferred embodiment of the invention, specic frequencies and component rarrangements have been described by way of example only; Those skilled in the art make numerous modifications of kand departures from the speciflc apparatus described herein without departing from thedisclosed inventive concepts. Consequently, the invention is to be construed as limited konly by the spirit and scope of the appended claims.

What isfclaimed is:

1`. A pulsed airborne Doppler radar system comprising, transmitting means'capable of earthward radiation of high frequency energy, receiving means capable of responding to said radiated high frequency energy which is returned from the surface of'the earth, said receiving means utilizing a iixed frequency signal from a source thereof,a source of a gating signal of frequency which is a selected subharmonic of said xed frequency, means responsive to said gating signal for activating said transmitting and receiving means respectively during alternating substantially equal time intervals, and means responsive to `stepwise system altitude changes for determining the subharmonic selected.

2. A pulsed Doppler radar system comprising, sources of coherently generated transmitted, local oscillator and gating signals, an antenna system responsive to an input signal for radiating a plurality of pencil beams and separately receiving energy returned from the respective radiated beams, associated with each beam a mixer with rst and second inputs, means` for `coupling the returned energy from each beam to a respective second input, and means responsive to said gating signal for applying said transmitted signal as an input signal to said antenna system and said local oscillator signal to the first input of each mixer during alternating mutually exclusive time intervals.

3. The apparatus of claim 2 wherein said alternating mutually exclusive` time intervals are of substantially equal duration. p 4. A pulsed 'Doppler radar system comprising,r a source of cohcrently` generated transmitted and local oscillator "signals,` an antenna system responsive to an input signal for radiating a plurality of pencil beams and separately receiving energy returned from the respective beams, associated witheach beam a mixer with iirst and second inputs, means forgcoupling the returned energy from each beam to a respective second input, a source of a gating signal, and 'means responsive to said gating signal for applying said transmitted signal as an input signal to said `antenna system` and said local oscillator signal to the rst input of each mixer during alternating mutually exclusive time intervals.

5. The apparatus ofclaim 4 wherein said mutually exclusive time intervals are of substantially equal duration.

6. A `pulsed Doppler airborne. radar system comprisfactor is two.

ing, sources of coherently generated transmitted, local oscillator and gating signals, an antenna system responsive to an input signal for radiating a plurality of pencil beams land separately receiving energy returned from the respective radiated beams, associated with each beam a mixer with rst and second inputs, means for coupling the returned energy from each beam to a respective second input, means responsive to said gating signal for applying said transmitted signal as an input signal to said antenna system and said local oscillator signal to the rst input of each mixer during alternating mutually exclusive substantially equal time intervals, altitudev sensitive apparatus, and means responsive to the altitude indication of said laltitude sensitive apparatus for controlling the frequency of said gating signal.

7. A pulsed Doppler airborne radar system comprising, sources of coherently generated transmitted, local oscillator and gating signals, anr antenna system responsive to an input signal for radiating a plurality of pencil beams and separately receiving energy returned from the respective radiated beams, Iassociated with each beam a mixer with first and second inputs, means for coupling the returned energy from each beam to a respective second input, means responsive to said gating signal for applying said transmitted signal as lan input signal to said antenna system and said local oscillator signal to therst input of each mixer during alternating mutually exclusive substantially equal time intervals, an altimeter, and means responsive to changes in the altitude indicated by said altimeter at selected altitudes for changing the frequency of said gating signal by an integr-al factor.

8. The apparatus of claim 7 wherein said integral Y 9. A pulsed airborne Doppler radar system comprising, transmitting means capable of earthward radiation of high frequency energy, receiving means capable of responding to said radiated high frequency energy which is returned from the surface of the earth, a source of a vperiodici gating signal, means responsive to said gaiting 4 signal for activating said transmitting and receiving means respectively during alternating time intervals, and means sensitive to the system altitude for controlling the period of said gating signal whereby the latter period is increased ordecreased in response to apredetermined increase or decrease respectively in the altitude sensed.

`10. A pulsedpairborne Doppler radar system comprisy ing, transmitting means lcapable of earthward radiation of high frequency energy, receiving means capable of responding to said radiated high frequency energy which f' is returned from the earth, said receiving means vutilizing 'a fixed frequency signal from a source thereof, a source g of a gating signal of frequency which is a selected subharmonic of Vsaid fixed frequency, means'respons'ive to y said gating signal for activating said transmitting and receiving means respectively during alternating substantially equal timeintervals,fand means sensitive to the system altitude'for selecting said subharmonic.

. 11. A pulsed airborne Doppler radar system comprising, transmitting means capable of earthward radiation of high frequency energy, receiving means capable of re- A spending to said radiated high frequencyenergy which is returned from the earth, said receiving means utilizing a iixedlfrequency signal kfrom a source thereof, a `source of a gating signal yof. frequency which is a selectedsubharmonic of said fixed frequency, vmeans responsive to vsaid gating signal for activating said transmitting and receivingmeans respectively during alternating substantially equal time intervals, and means responsive 1to an altimeter for changing the order of said subharmonic'in response v*to predetermined change in-,the indicated altitude.

. 12. A pulsed airborne Dopplery radar system comprising-transmitting means capable of earthward radiation of` high frequency energy, receiving means capable `of L `ifesponding to ysaid radiated high frequency energy which is'reected, said receiving ymeans, utilizing. a fixed freto the altimeter indication for activating one of saidv gates, buffer means for coupling the output signal from the activated gate to a gating nip-flop which responds to `the latter with oppositely phased square-Wave gating signals each of frequency which is a subharrnonic of said fixed frequency -selected in accordance with the altimeter activated gate whereby indication of the next higher altitude range effects the selection of the next lower frequency subh-armonic.

13. A pulsed airborne Doppler radar system comprising, transmitting means capable of earthward radiation of high frequency energy, receiving means capable of responding to said radiated high frequency energy which is returned from the surface of the earth, gating means for activating said transmitting and receiving means during alternating time intervals of equall duration, and altitude sensing means for causing said gating means to activate said transmitting and receiving means at a rate inversely proportional to the altitude of said system.

14. A pulsed airborne Doppler radar system in accordance with claim 13 wherein said altitude sensing means is arranged to vary said rate in predetermined steps in response to respective stepwise variations in the altitude of said system. Y

15. A pulsed radar system comprising, transmitting means capable of illuminating a surface with high frequency energy, receiving means capable of responding to the radiated high frequency energy which is returned from said surface, a source of a periodic gating signal, means responsive to said gating signal for activating said transmitting and kreceiving means respectively during alternating time intervals, and distance sensing means responsive to the distanceA between the transmitting and receiving means and said surface forA controlling the period of said gating signal whereby the latter period is increased or decreased stepwise in response to a predetermined stepwise increase or decrease respectively in the distance sensed. v

E16. A pulsed radar system comprising, transmitting means capable of illuminating a surface with high fremeans responsive to said gating signal-for activating said transmitting and receiving means respectively during valternating time intervals, `and distance sensing means responsive to the distancebetween the transmitting and receiving means and said surface for controlling the period of said gating signal whereby ther latter period" is increased or decreased in response to a predetermined increase or decrease respectively in the distance sensed.

17, An airborne pulsed radar systeml comprising, a source of a constant lhigh frequency signal, transmitting means for radiating said signal during'alternate-ones of equal time intervals, gating means for controlling the VVduration ofsaid intervals, a signal receiver, said gating means causing said receiver to be responsive toreflections of the radiated signal only during the nonradiating intervals of said transmitting means, .and altitudevjsensing means governing the duration of said'int'ervalsfso that saidtransmitting means ceases to radiate just prior to. A. 1 ther receptionl of the frontpof; the ree'cted signal. f f

References Cited in the -x'ile of thispatent.v Y

` l'(.ITED STATES, PATENTS Ginzton Octyzs, 1949 Armstrong -...j....a.'...-v Mar. 13, 1956 Kockv Mayl 14, 1946 

